This invention relates generally to object detectors and particularly to detectors for sensing the presence and absence of documents, images or microfilm, and the like.
In the past, document detectors have been provided for sensing the presence of a document as it passes between a light source and a photosensitive device. The interruption of the light by the document results in a change in the light received by the photosensitive device and a corresponding change in the flow of current therein. Typically, the change in the flow of current through the photosensitive device has been converted to a voltage change for developing a pulse indicative of the presence of a document. The number of pulses can then be counted to determine the number of documents which pass the detector. Similar systems operate by sensing the light reflected by a document as it passes along a document path adjacent the photosensitive device.
A problem which has been addressed and solved by a prior document detector is that of maintaining the voltage which is derived from the current output of the photosensitive device at a known and constant level under the condition existing when no document is passing the detector, regardless of long term drifts in the output of the light source, changes in the characteristics of the photosensitive device, or the like. Thus, despite any long term change in the current flowing in the photosensitive device, the voltage derived from that current, referred to herein as the image location voltage, is held at a known reference level, typically by a feedback loop. Accordingly, only the abrupt and substantial change in the flow of current in the photosensitive device which occurs when a document passes the detector results in a change in the image location voltage and only then is an indication generated of the passage of a document past the detector.
Although prior detectors do reliably accommodate long term drifts in the system to establish the image location voltage at a constant reference level as an indication that no document is being detected, they rely merely on a large change in the level of the image location voltage as an indication of the presence of a document. Such prior detectors are, however, incapable of maintaining the image location voltage at a second predetermined reference level in the presence of a document. Consequently, the latter level of the image location voltage is subject to change. For example, should a document remain in the detector and interrupt the light source for an indefinite period of time, the level of the image location voltage may change with drifts in the characteristics of the light source or the photosensitive device. The resulting indefiniteness in the level of the reference voltage under this latter condition is particularly disadvantageous in systems such as microfilm readers in which a microfilm bearing images is advanced past the light source for generating a pulse each time an image interrupts the light source. Frequently, the film may be stopped for an indefinite time at a point where the light source is interrupted by an image. Under this condition, it is desirable that the detector be able to hold the image location voltage at a predetermined level as long as the image interrupts the light source, regardless of any drifts in the system or gradual changes in ambient light, in order to generate a positive indication that an image is being detected. Of course, it is also required that the image location voltage be held at another predetermined reference level when no image is interrupting the light source. Detectors which maintain a constant reference voltage level only under one of the two conditions which are frequently encountered in microfilm readers, i.e. the indefinite absence or presence of an image, are, accordingly, subject to developing an indeterminate output when a document or image remains detected for a prolonged duration.
Accordingly, it is an object of this invention to provide an improved control system for an image detector.
It is a more specific object of this invention to provide a control system which maintains the image location voltage at a first predetermined reference level under the condition of prolonged presence of an image and at a second predetermined reference level under the condition of prolonged absence of an image, despite long term changes in ambient conditions or detector component values.